The mechanism of toxicity for organophosphorus (OP) anticholinesterases involves steps which may be selectively affected by some, providing a basis for differential toxicity. Acetylcholine release and its regulation by inhibitory muscarinic autoreceptors may be pivotally important in OP toxicant-selective toxicity. Some anticholinesterases (including the oxons of both parathion and chlorpyrifos, i.e., paraoxon and chlorpyrifos oxon) directly interact with muscarinic M2 receptors, the receptor subtype of muscarinic autoreceptors. Using superfused brain slices to measure acetylcholine release, both qualitative differences in the in vitro effects of paraoxon and chlorpyrifos oxon and time-dependent differences in the alteration of muscarinic autoreceptor function following in vivo parathion and chlorpyrifos exposures were noted. It is hypothesized that muscarinic autoreceptor function is an important modifier of cholinergic toxicity and that selective modulation or age-related differences in its activity can lead to differential toxicity. In aim 1, the dose-related effects of parathion and chlorpyrifos on cholinergic toxicity and acetylcholine release in vivo using microdialysis techniques will be compared in adult rats. Aged rats appear more sensitive to the acute effects of both chlorpyrifos and parathion. Studies in aim 2 will evaluate whether acetylcholine release/autoreceptor function may contribute to aging-related differences in OP insecticide toxicity. Aim 3 will evaluate the consequences of prior modulation of autoreceptor function (by infusion of autoreceptor agonist, antagonist or M2 receptor antisense) on paraoxon and chlorpyrifos oxon toxicity in adult rats. G-protein receptor kinase (GRK)-dependent phosphorylation of M2 receptors initiates receptor desensitization, internalization and down-regulation. Studies in Aim 4 will test the hypothesis that some OP toxicants differentially alter GRK-mediated phosphorylation of M2 receptors leading to selective changes in receptor regulation. These studies should clarify the selective effects of OP insecticides on acetylcholine release/autoreceptor function in vivo, determine the relative role of autoreceptor function in age-related sensitivity, and evaluate whether alteration of GRK-mediated M2 regulatory pathways contribute to the selective modulation of muscarinic autoreceptor function by some OP toxicants.